An electric power assist system, which generates torque electrically, is used more and more in recent years as a mechanism for assisting the steering operation of a steering wheel of a vehicle. As an electric motor as a power generation source of the electric power assist system, a brushless motor is used for example. The brushless motor is driven to rotate by application of three-phase alternating currents. It is necessary to generate the alternating currents of different phases from a direct current power source of a predetermined voltage (for example, 12 volts) so that coil currents of different phases are supplied to coils of a plurality of phases (for example three phases). An electronic circuit is thus needed to switch over the coil currents. The electronic circuit includes a semiconductor module, a noise filtering aluminum electrolytic capacitor, a noise filtering choke coil, a microcomputer and the like. A semiconductor chip, which performs the switching operation, is integrated within the semiconductor module. It is proposed to arrange the electronic circuit near the motor (for example, patent document 1).    Patent document: JP 2002-345211A
A comparatively large motor is normally used in the electric power assist system to provide sufficient torque. Further, a large aluminum electrolytic capacitor and a choke coil are normally provided on the electronic circuit to protect the semiconductor module from being broken by surge voltages generated by the current switching operation and to reduce electric power source noise.
A variety of systems are also provided recently in a vehicle in addition to the electric power assist system. These systems need respective spaces in the vehicle to be mounted. The motor itself of the electric power assist system is thus required to be smaller in size. According to the motor disclosed in the patent document 1, the semiconductor module and electronic parts such as the aluminum electrolytic capacitor are arranged in the axial direction of the motor.
The choke coil has a comparatively large volume and occupies a large space in case it is mounted on a printed circuit board of the electronic circuit. Even if the choke coil is ring-shaped and arranged perpendicularly to the printed circuit board, it is likely that its winging becomes non-uniform partly and magnetic field leaks. The choke coil is provided in a power cable of the semiconductor module. Since a large current flows in the choke coil, the choke coil must be arranged to promote heat dissipation.